Ionization vacuum pump of the orbitron type having a porous annular grid electrode



Aug. '29, 1967 3,339,106 A POROUS P. A. REDHEAD IONIZATION VACUUM PUMP OF THE ORBITRON TYPE HAVING ANNULAR GRID ELECTRODE Flled May 28, 1965 PA A. P594040 8 PATF/VT A cwr INVENTOR United States Patent ABSTRACT OF THE DISCLOSURE An ionization vacuum pump. of the orbitron type in which the ionization and gettering functions are separated. A generally open grid surrounding the central anode forms an ionization region therebetween and an outer gettering or sorption region in the annular space between it and the inner metallic surface of the envelope which acts as ion collector. The grid has a potential applied to it which imparts increased energy to ions formedin the ionization region accelerating them towards the collector.

This invention relates to an ionization vacuum pump.

In most ionization pumps the long electron paths necessary for efficient ionization are achieved by allowing the electrons to orbit in a magnetic field. In the orbitron pump, a device first proposed by Professor D. Gabor and for which Canadian Patent No. 663,569 issued on May 21, 1963, electrons from a thermionic cathode are trapped and orbit in the electric field between two coaxial cylinders. Angular momentum is conserved under these conditions of a central field, and long electron paths are obtained which are treated theoretically by R. H. Hooverman in the Journal of Applied Physics, 34:3503; 1963.

The orbitron pump is attractive for many uses as its pumping speeds approach those of the magnetic type pumps (Penning pumps) without having the great disadvantage of requiring heavy and cumbersome magnetic field apparatus.

In the orbitron devices built to date the outer cylinder (collector) is solid and the maximum sticking probability obtained (for argon) is about 0.6. In addition, the getter material (titanium) is evaporated by electron bombardment of a titanium cylinder fixed to the central anode. An orbitron of this form is described by Mourad, Pauly and Herb in the Review of Scientific Instruments for June 1964. volume 35, number 6.

It is an object of the present invention to provide an ionization pump of the orbitron type wherein the pumping speed is greatly increased and the power dissipation reduced in relation to pumping speed.

This and other objects of the invention are achieved by providing an ionization pump of the orbitron type wherein the outer solid cylinder is replaced by an open grid allowing an extra accelerating potential to be applied to the ions before their capture on a layer of getter material positioned externally of the said grid thus greatly increasing the sticking probability of the ions when they strike the said layer. In addition, a separate source of getter material is placed outside the said grid and heated resistively to provide the getter film. The absence of a central getter cylinder on the anode allows longer electron path lengths.

In drawings which illustrate an embodiment of the invention,

FIGURE 1 is a vertical cross-section of an ionization pump of the stated type and,

FIGURE 2 is a horizontal cross-section taken on the line A-A of FIGURE -1.

Referring to FIGURE 1, an ionization pump is formed 3,339,106 Patented Aug. 29, 1967 of an envelope 1 of glass (metal may be used) which is provided at one end with an opening 2 for connecting the pump to a system to be evacuated. At the other end 3 of the pump a series of electrical lead-ins pass through the envelope in the standard manner. The inside of the glass envelope 1 is coated with a layer 4 of tin oxide and forms the final target for absorption or adsorption (trapping) of the ions. Electrical contact with this layer is made by means of lead-in 18 passing through the envelope via glass seal 17. An anode in the form of a thin wire 5 is positioned centrally of the interior of the pump and is connected to lead-in rod 6 which acts as a support as well as an electrical connector. A shadow-plate 12 is positioned on lead-in rod 6 to prevent the formation of a titanium film around lead-in rod 6 which would lead to electrical leakage. A grid made up of a series of metal rings 16 mounted on rods 13 surrounds the anode. Rods 13 are connected to the exterior by means of connectors 14. A mesh or highly perforated cylinder could be used for the grid as well. The end of the grid is formed by a mesh 21. A filament 10 for producing ionizing electrons is connected to lead-in rod -11 and also to a plate-like structure 15 mounted on rods 7 and 8. Apertures are formed in plate 15 for the passage of grid rods 13 and filament 5 with a sleeve 9 being provided to shield the anode from the filament. Getter material in the form of a composite titanium-tungsten ring 19 is positioned outside the grid. This ring is supported on rods 20 shown in incomplete fashion in FIGURE 1 but readily seen in FIGURE 2.

FIGURE 2 is a cross-section of the pump on line AA and shows the positioning of the elements (which carry the same reference numbers) in the transverse plane.

In operation, the pump is suitably connected to a system to be evacuated, a suitable heating current is applied to filament 10 and electrons are emitted into the region between the anode and the grid. The electrons, due to their momentum and the radial electric field existing between the grid and the anode produced by a potential difierence (Vga) between these electrodes, orbit around the anode. It has also been observed that these paths displace in the axial direction and oscillate back and forth between certain boundaries, in this case, end plate 15 and mesh 21 which forms the end of the grid. As a result of these motions very long electron paths are obtained. The electrons in their flight strike molecules of the gas forming positive ions. These ions are accelerated towards the grid but do not stop there. Because of a voltage potential (V applied between the grid and the outer coated walls (ion collector) an extra accelerating force is applied to the ions and they are driven into the titanium film with high energy greatly increasing the sticking factor. The titanium film is produced by resistively heating titanium-tungsten ring 19 and evaporating titanium metal onto the surface of the envelope forming a fresh surface for entrapment of the ions.

In a pump built according to the above description, it was found that with the voltage applied between grid and anode (V equal to 2.0 kv. and the voltage between grid and ion collector (V equal to 1.0 kv., a pumping speed for argon of 0.48 litre/second was obtained. The sticking probability of the ions was found to be very close to 1.00.

In the orbitron pumps presently being built the ionization and the gettering functions take place in the same region. In the pump described here these two functions have been separated and take place in separate regions. By doing this it is possible to impart additional energy to the ions before they strike the ion collector and also it allows the removal of the gettering material source from the central region to an external position where it does not interfere with the electron paths. As a result of these modifications, the efficiency in terms of increased sticking probability and pumping speeds for the orbitron pump is increased.

What is claimed is:

1. An ionization vacuum pump of the orbitron type comprising:

(a) an anode,

(b) a generally open grid surrounding said anode to form an ionization region therebetween,

(c) an ion collector having a solid surface positioned outwardly of said grid forming a sorption region between it and said grid,

(d) a source of getter material positioned inwardly of said ion collector,

(e) an electron emitting filament positioned in said ionization region,

(f) means for applying a potential difference between the anode and grid to produce an electric field such that electrons from the filament will orbit in the ionization region, and

(g) means for applying an accelerating potential between grid and ion collector to impart increased energy to ions formed in the ionization region accelerating them towards said ion collector.

2. An ionization vacuum pump as in claim 1 wherein said grid is in the form of a series of spaced rings mounted on a supporting structure.

3. An ionization vacuum pump as in claim 1 wherein said grid is a cylindrical mesh screen.

4. An ionization vacuum pump as in claim 1 wherein said grid is a preforated cylindrical metal shell.

References Cited UNITED STATES PATENTS 2,653,620 9/1953 Morgan 324-33 X 2,980,317 1/1961 Reich. 3,051,868 8/1962 Redhead 324-33 X 3,267,326 8/ 1966 Hayward 315l08 X FOREIGN PATENTS 887,251 1/ 1962 Great Britain.

JAMES W. LAWRENCE, Primary Examiner. STANLEY D. SCHLOSSER, Assistant Examiner. 

1. AN IONIZATION VACUUM PUMP OF THE "ORBITRON" TYPE COMPRISING: (A) AN ANODE, (B) A GENERALLY OPEN GRID SURROUNDING SAID ANODE TO FORM AN IONIZATION REGION THEREBETWEEN, (C) AN ION COLLECTOR HAVING A SOLID SURFACE POSITIONED OUTWARDLY OF SAID GRID FORMING A SORPTION REGION BETWEEN IT AND SAID GRID, (D) A SOURCE OF GETTER MATERIAL POSITIONED INWARDLY OF SAID ION COLLECTOR, (E) AN ELECTRON EMITTING FILAMENT POSITIONED IN SAID IONIZATION REGION, (F) MEANS FOR APPLYING A POTENTIAL DIFFERENCE BETWEEN THE ANODE AND GRID TO PRODUCE AN ELECTRIC FIELD SUCH THAT ELECTRONS FROM THE FILAMENT WILL ORBIT IN THE IONIZATION REGION, AND (G) MEANS FOR APPLYING AN ACCELERATING POTENTIAL BETWEEN GRID AND ION COLLECTOR TO IMPART INCREASED ENERGY TO IONS FORMED IN THE IONIZATION REGION ACCELERATING THEM TOWARDS SAID ION COLLECTOR. 